The phenomenal growth in the demand for radio communications has put persistent pressure on radio communication network operators to improve the capacity of their networks. To improve the efficiency of these networks, scarce radio resources have to be reused aggressively in neighboring cells. As a result, inter-cell interference has become a main source of signal disturbance, limiting not only the service quality to user equipments (UEs) at the cell edges, but also the overall system throughput.
Inter-cell Interference Coordination (ICIC) is one technique for mitigating downlink inter-cell interference. According to ICIC, base stations communicate scheduling and/or interference information amongst themselves. An “aggressor” base station that generally contributes to interference at user equipments (UEs) served by a “victim” base station, for instance, sends information to the victim base station describing the radio resources on which the aggressor base station will cause interference. This way, the victim base station can intelligently schedule downlink transmissions to user equipments (UEs) so as to avoid that interference.
Consider, for example, a radio communication network based on Long Term Evolution (LTE) standards that employs ICIC. An aggressor base station (i.e., eNodeB) sends a LOAD INFORMATION message over a logical X2 interface to a victim base station. A LOAD INFORMATION message in this regard includes a so-called Relative Narrowband Transmit Power (RNTP) information element. The RNTP information element includes an RNTP bitmap. Each bit of the RNTP bitmap corresponds to a respective downlink resource, defined as a physical resource block in LTE (i.e., one 0.5 ms slot in the time domain and 12 contiguous subcarriers in the frequency domain). The value of a given bit indicates whether the aggressor base station promises to protect the corresponding downlink resource from interference by not transmitting on that resource with a transmit power higher than a certain threshold. This threshold is appropriately called the RNTP threshold and is also included in the message. Based on the RNTP bitmap, the victim base station may avoid scheduling downlink transmissions to UEs (at least UEs subject to high interference, such as those at the cell edge) on certain downlink resources; namely, those on which the aggressor base station may transmit with a power higher than the RNTP threshold.
Potential values of the RNTP threshold include −∞, −11, −10, −9, −8, −7, −6, −5, −4, −3, −2, −1, 0, +1, +2, and +3 dB. For example, if the aggressor base station sets the RNTP threshold to be −∞, the aggressor base station promises that it won't transmit any data on downlink resources that correspond to bits in the RNTP bitmap with a value of “0.” Alternatively, if the aggressor base station sets the RNTP threshold to be 0 dB, the aggressor base station promises that it will transmit less than maximum power on downlink resources that correspond to bits in the RNTP bitmap with a value of “0.” As yet another example, if the aggressor base station sets the RNTP threshold to be +3, the aggressor base station indicates that it will use power boosting up to 50% higher than maximum power on downlink resources corresponding to bits in the RNTP bitmap with a value of “1.” Regardless of the indication by the aggressor base station, the victim base station assumes the indication remains the same until it receives a subsequent RNTP bitmap in the future.
One non-limiting context in which downlink interference coordination proves especially effective is in a radio communication network that employs low power base stations, such as pico base stations, home base stations, relays, etc. These low power base stations are strategically deployed alongside high power (i.e., macro) base stations to add additional capacity to areas of high traffic and/or to improve the coverage of areas with otherwise bad coverage. The radio network is thus appropriately referred to as a heterogeneous or multilayer network. In this setting, the macro base stations appear as aggressor base stations to the low power base stations. But, with the macro base stations sending scheduling and/or interference information to the low power base stations, as described above, the low power base stations are able to intelligently schedule downlink transmissions so as to avoid macro interference.
Although downlink interference coordination proves largely effective in this and other contexts, such coordination still has limitations.